The present invention relates to a method for the promotion of white solidification of nodular or vermicular cast iron samples which is required for the determination of the carbon content and the carbon equivalent of such samples from the study of their duly recorded cooling curves. It is common practice to sample molten iron or steel for studying or controlling some properties of the molten metal.
It is already a common practice to determine some properties of molten iron from the study of the duly recorded cooling curves. This method is based upon the fact that there is a relation between the temperature corresponding to the liquidus arrest of a casting and its carbon equivalent. More recently, it has been determined that there is also a relation between the liquidus and metastable solidus temperatures and the carbon content of an iron sample (Foundry, Management and Technology, July 1974, pages 80 to 83). The eutectic austenitecementite temperature is generally called the solidus temperature.
The exact and clear detection of solidus and liquidus levels in a cooling curve of a hypereutectic iron sample requires that the the sample solidifies white. For this reason, the known method mentioned hereabove could until now only be applied to hypoeutectic and non-nodular and non-vermicular hypereutectic castings.
As far as the analysis of hypereutectic castings is concerned, it is already known (U.S. Pat. No. 3,546,921) to add to samples an element that is able to stabilize the carbide in order to delay the graphite formation. Such an element can be tellurium, bismuth, cerium, magnesium and the like, and is added in a quantity not surpassing 0.40% in weight of the sample.
The interaction of magnesium with tellurium when treating the casting was the object of a study published in the journal Russian Castings Production (1970, 3, pages 146-147) from which it appears that tellurium, even when added in small quantities to the casting, can have a denodularisation effect due to its combination with magnesium. On the other hand, the inhibitory effect of tellurium and of selenium, as well as of sulphur upon the formation of spheroidal graphite in castings has recently been described in the journal Imono (47, 1976, 12, 836). None of the above-mentioned studies however discloses how to obtain white solidification of a nodular or vermicular casting in a way which is easy and reliable.
It should be noted that not only the study of the cooling curve but also analysis with an emission-spectrometer requires the white solidification of a sample of such a molten iron.
The aim of the invention is to provide a solution to the problem relating to the obtainment of such a type of solidification.